The goal of GenPharm's program is to develop methods for the introduction of yeast artificial chromosomes (YACs) into the germline of mice, and to apply the technology toward the generation of transgenic mice expressing a broad, fully human immunoglobulin repertoire. These mice would permit the isolation of human monoclonal antibodies in an analogous manner to rodent monoclonal antibodies. In Phase I, methods for the introduction of YACs into mice were developed, and mice carrying a small YAC encoding a minimally competent human immunoglobulin transgene were shown to express human heavy chain IgM antibodies in the serum. In Phase II, the animals generated in Phase I will be fully characterized for transgene function and expression. Additional constant and variable regions will be cloned in YACs and added to the Phase I YAC to permit class switching, somatic mutation, and IgGproduction. These Phase II transgenes will be introduced into mice using methodology developed in Phase I. Transgenic mice will also be generated carrying large fragments of the unrearranged human kappa gene and bred to the Phase II heavy chain mice. These mice will be bred with mice lacking functional immunoglobulin genes to produce mice demonstrating a broad repertoire of high affinity, fully human antibodies.